In recent years, fuel cells with high efficiency and high environmental performance are being desired. However, fuel cells are not still widespread for high production costs. One of the reasons of high production costs is a high price of a fuel cell separator. In conventional fuel cell separators, since gas flow channels are formed on metallic flat plates by machining or etching process, production time is lengthy and the production unit price for a piece becomes extremely expensive.
Therefore, reduction of production cost of a fuel cell separator is important in order to spread usage of fuel cells. As a result, processing methods of metallic fuel cell separators are moving towards press forming from expensive etching, etc. However, an issue exists in that when a plurality of concavoconvex parts is press-formed on a smooth metallic flat plate, residual stress remains on the concavoconvex parts and the periphery thereof, and a warp occurs on the fuel cell separator.
If a warp occurs on a fuel cell separator, an issue emerges in that the surface pressure between the fuel cell separator and electrodes becomes uneven and the contact resistance becomes high, and reduction of gas sealing performance along with lowering generated voltage takes place. Furthermore, there is also an issue in that since fuel cell stacks need to be assembled while correcting warps, the assembling work becomes complicated.
Also, for fuel cells used in fuel cell vehicles, since smaller and lighter fuel cells are desired, fuel cell separators are produced with thin metallic flat plates of approximately 0.2 mm to 0.3 mm in thickness. However, since thickness of the metallic plate is thin and the stiffness is low, when gas flow channels are press-formed, a warp tends to occur on the fuel cell separator. Then, technologies to reduce a warp even on a fuel cell separator of thin thickness are disclosed in Patent Literature 1 and Patent Literature 2.
In the technology of Patent Literature 1, the technology of a fuel cell separator, composed of formed conductive parts in the center of a metallic flat plate and edge parts on the periphery of the conductive parts, is disclosed. According to the technology of Patent Literature 1, stiffness of the edge parts becomes high by forming ribs with convex parts on one side and grooves on the other side on the edge parts of the conductive parts, thereby a warp of the entire fuel cell separator is suppressed.
However, in the technology of Patent Literature 1, providing the ribs on the edge parts is an essential requirement, since the ribs raise stiffness of the edge parts and a warp of the fuel cell separator is suppressed. As a result, a fuel cell separator with entire edge parts being flat cannot be given. In this case, since passage holes arranged in a manner to penetrate flat parts of the edge parts while overlapping cells for such as fuel flow are constrained in placement, issues emerge such as large surface area of a cell, necessity of providing concavoconvex grooves to match the shapes of the ribs, along with larger thickness of sealing material.
In Patent Literature 2, the technology is disclosed to suppress a warp on a solid polymer type fuel cell separator of less than 0.2 mm thin plate. According to the technology of Patent Literature 2, in a fuel cell separator comprising gas flow channels along the fuel cell separator in the center and flat parts on the periphery thereof, bent parts or concave or convex parts other than the gas flow channels are formed on two sides of the flat parts parallel to the longitudinal direction of the gas flow channels. Meanwhile, portions without the bent parts, etc. are prepared on two sides of the flat parts parallel to the wide direction of the gas flow channels. With this, it is asserted that a warp of the fuel cell separator can be suppressed by increasing stiffness of the flat parts, even if thin plate of less than 0.2 mm, which is difficult to assure stiffness as material for fuel cell separator components, is employed.
However, according to the technology of Patent Literature 2, since a thin plate of less than 0.2 mm in thickness, which is difficult to assure stiffness, is targeted, an issue existed in that partial damage and warps might occur by collisions of the plates while moving the plates in a laminating assembly process. Furthermore, in the case of a thick plate, by judging to assure stiffness enough for a fuel cell separator as material, occurrence of a warp was not projected.
On the other hand, in the case of fuel cells utilized as stationary power generators, which are not necessarily required to be smaller and lighter as in fuel cell vehicles, a fuel cell separator of a thick plate was possible to be used. Here, the thick plate means a metallic plate with at least more than approximately 0.5 mm in thickness.
However, when a thick plate is employed for a fuel cell separator, although the fuel cell separator is not susceptible to deform and is easy to assemble at the time of stacking the fuel cell separators, once the thick plate was warped, conventional suppression methods could not suppress the warp.
The inventor of the present invention made prototypes by using plates of 0.8 mm in thickness considered not susceptible to warp, at the time of press-forming fuel cell separators. However, even if a thick plate is employed for a fuel cell separator, the inventor faced a new issue in that a warp may occur on the fuel cell separator depending on density and arrangement of concavoconvex parts.
In such a case, since stiffness of the plate itself is high in addition to strong residual stress acting on the thick plate in the magnitude to cause a warp, it was extremely difficult to correct the warp. Therefore, the inventor of the present invention challenged to provide a production method of a fuel cell separator to be able to correct a warp, even if the warp occurs on a thick metallic plate when concavoconvex parts are press-formed in high density, and a fuel cell separator with a warp corrected.
In Patent Literature 3, the technology of a production method of a fuel cell separator, in which a warp incurred by press forming on the fuel cell separator can be corrected by applying pressure to and partially compressing flat parts of the periphery of gas flow channels after press-forming the gas flow channels on a metallic thin plate, is disclosed. According to the technology of Patent Literature 3, in the first step, the gas flow channels composed of vertically long concavoconvex shapes are press-formed in the center, leaving the periphery up and down and sideways of the metallic flat plate. And, in the second step, a plurality of compression-formed parts, which are parallel to the longitudinal direction of the gas flow channels and also continuously linear from the ends of the gas flow channels to the peripheral edges of the fuel cell separator, are press-formed contiguously only on the flat parts of the edge parts in the longitudinal direction of the gas flow channels.
Although the compression-formed parts are said to be extremely shallow compressed parts with approximately 1/1000 plate thickness, forming concavoconvex shapes on the peripheral flat parts is unchanged, and issues emerged in that the assembling work was difficult while maintaining airtight and watertight properties of the fuel cell separator, and deterioration of airtight and watertight properties was possible with a lapse of time.
In Patent Literature 4, the technology of a production method of a fuel cell separator, in which a warp incurred by press forming on the fuel cell separator can be corrected by applying tensional force on the periphery of gas flow channels after the gas flow channels are press-formed on a metallic thin plate, is disclosed. According to the technology of Patent Literature 4, in the first step, the gas flow channels of vertically long concavoconvex shapes are press-formed in the center, leaving the periphery up and down and sideways on the metallic flat plate. And, in the second step, while the periphery parallel to the longitudinal direction of the above gas flow channels is fixed firmly, tensional force is applied only to the periphery orthogonal to the longitudinal direction of the gas flow channels to correct a warp of the fuel cell separator.
Specifically, flat parts on both sides of the gas flow channels on the fuel cell separator are bent downwards to create hooking pieces, the periphery of the gas flow channels is fixed firmly by pinching from top and bottom with jigs, cam dies engage onto the hooking pieces on both sides of the gas flow channels, and the both sides of the gas flow channels are spread by opening the cum dies sideways. Therefore, fixing mechanism to vertically pinch the fuel cell separator, and cum die mechanism to spread horizontally the both sides of the fuel cell separator are required.
Then, in the technology of Patent Literature 4, issues existed in that since equipping the above cum die mechanism to a press machine was necessary, the press mechanism became complex and spreading only the above both sides by using a conventional press machine was difficult. Therefore, the inventor of the present invention made trial to correct a warp of a thick plate with a warp occurred only by using a press mechanism of a conventional press machine. In the course of the trial, for a thick plate, only the both sides elongated, and correcting a warp on concavoconvex parts could not be achieved by spreading only the above both sides.